1. Field of the Invention
The present invention relates to a thermostat which does not abrade a monitored object and is superior in heat-responsiveness.
2. Description of the Related Art
Copiers and printers have a fixing roller rolled in a heated state for fixing ink onto printing paper. In order to achieve the secured fixation, a fixing roller is in contact with printing paper. These apparatus also have a thermostat in contact with a fixing roller to prevent a fixing roller from overheating.
When, therefore, a fixing roller overheats due to a malfunction of a temperature control unit such as a thermistor, and reaches a predetermined temperature, a thermosensitive member of a thermostat recurves and interrupts the electrical power. This results in preventing the fixing roller from smoking. The contact surfaces between a thermostat and a fixing roller are coated with a resin film to avoid the latter being abraded.
The drawback of the above conventional thermostat is the additional process of coating it with a resin film in its manufacturing. Even though the coating process is added, there still remains the abrasion problem because a fixing roller is in constant contact with a thermostat. While this problem can be solved by keeping them out of contact, this solution has the critical disadvantage of the deterioration of the heat conductivity and responsiveness for avoiding overheating of a thermostat.
Copiers and printers these days are designed to start operation immediately after electricity is turned on and heat a fixing roller quickly. The above bad heat-responsiveness could cause some troubles such as smoking when a thermostat is operated.
Therefore, it is an object of the present invention to provide a thermostat which does not abrade a monitored object, even if rotatable or movable, and is superior in heat-responsiveness. To achieve the above object, the present invention comprises an approximately round thermosensitive member, a holder for holding one surface of said thermosensitive member and a cover member for holding the other surface of said thermosensitive member, wherein said thermosensitive member recurves when it exceeds a critical temperature, and said cover member has an opening which does not hinder said cover member from holding said thermosensitive member, and portions projecting radially inward from said opening. The opening is not limited to a particular configuration and typically formed in a circular or oval shape, depending on the configuration of a thermosensitive member. If a circular opening is adopted, it is preferable that an opening with a diameter larger than that of a thermosensitive member is formed and said projecting portion alone hold the thermosensitive member.
The present invention also comprises an approximately polygonal thermosensitive member, a holder for holding one surface of said thermosensitive member and a cover member for holding the other surface of said thermosensitive member, wherein said thermosensitive member recurves when it exceeds a critical temperature, and said cover member has an opening which does not hinder said cover member from holding said thermosensitive member and holds said polygonal thermosensitive member on all angles or at least one angle.
In this invention, the cover member has the opening with such shape and diameter as not to hinder the cover member from holding said thermosensitive member. This puts the thermosensitive member in direct contact with the air heated by the fixing roller or the like and with the radiant heat emitted from the fixing roller or the like. This keeps its high heat responsiveness. And in this invention, the thermosensitive member is held by portions of the cover member projecting radially inward, and on all or a part of the angles of its approximately polygonal shape. This reduces the loss of heat conductivity from the thermosensitive member to the cover member and also keeps the high heat responsiveness of the former.
In this invention, holding one or the other side of the thermosensitive member means not only holding by putting it in direct contact with the cover member or holder but also holding in such a way as to prevent the thermosensitive member from being apart from the thermostat. When the thermosensitive member is disposed perpendicular downward to the cover member, the two members do not have to be in contact with each other. On the contrary, when the thermostat is made upside down and the thermosensitive member is disposed perpendicular upward to the cover member, the holder and the thermosensitive member do not have to be in contact with each other.
In a preferred arrangement, the surface opposite the cover member of the thermosensitive member is treated to absorb heat. The treatment method is not particularly limited. It is typical to coat the surface with heat-resistant paint superior in heat absorption. Black paint is listed as paint superior in heat absorption. If the thermosensitive member is treated to absorb heat, its heat responsiveness becomes still better. Since most thermostats are actuated at a maximum of about 300xc2x0 C., it is preferable to adopt heat-resistant paint which does not deteriorate due to heat beyond that degree.
It is preferable that the thickness of paint is less than 30xcexc. Beyond that, a firm paint film hinders the thermosensitive member from recurving. Even worse, when the thermosensitive member recurves repeatedly, a paint film may be stripped off its surface. After many data were analyzed and economical efficiency considered, it was concluded that as for heat-resistant paint, its thickness is preferably about 4 to 6 microns regardless of the kind.
The surface of the thermosensitive member, preferably the whole surface, is exposed through the opening of the cover member. Only the holding portion of the cover member preferably covers the thermosensitive member.
The thermosensitive member may be formed in a round shape or an approximately polygonal shape. If the thermosensitive member is to be formed in an approximately polygonal shape, an approximately rectangular shape is preferable. In this arrangement, the curvature radius of the thermosensitive member in the longitudinal direction is formed longer than that of the latitudinal direction. For example, when a monitored object is in an axial shape, such as a fixing roller, more sufficient heat-absorbing operation can be achieved by making the longitudinal direction of the thermosensitive member parallel to that of the monitored object. In addition, the curvature radius in the latitudinal direction of the thermosensitive member is formed enough to make the pin move and turn on the switch.
As for the cover member of this invention, it is preferable that the portion holding the thermosensitive member projects towards the thermosensitive member. In such arrangement, the air heated by the fixing roller and the like can heat the thermosensitive member more quickly, reaching the reverse side of the thermosensitive member through the non-projecting portion of the cover member. One surface of the thermosensitive member has higher heat expansion rate and the other has the lower rate, and the above arrangement is preferable, especially when the surface with the lower rate faces a monitored object (a fixing roller, etc.).
The holder of this invention has preferably one or more projections which project towards the thermosensitive member, which is held by this projection. While escape of heat absorbed by the thermosensitive member to other members reduces its heat responsiveness to that extent, the above arrangement can improve the heat responsiveness because of a decreased contact area between the thermosensitive member and the cover member. Because the air heated by the fixing roller etc. moves to the reverse side of the thermosensitive member through the non-projecting portion of the holder, it can heat the thermosensitive member more quickly.
In general, the pin which transmits the recurving operation of the thermosensitive member to a contact portion is built in the thermostat, and a through hole which guides the pin is formed in the holder. Preferably through portions are formed radially outside the through hole for the pin. In this arrangement, as heat transmission from the thermosensitive member to the holder is hindered, the heat responsiveness of the thermosensitive member is improved.
Moreover, the heat which escapes from the thermosensitive member by contact with other members is reduced if the other members have poor thermal conductivity. A poor heat-conducting material is preferably elected for the cover member, typically stainless steel.